Abstract

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A major lithospheric boundary is preserved within the western Idaho Batholith. The juxtaposition of two suites of supracrustal rocks, exposed as sheets within intrusive rocks, is the expression of this boundary at the level of exposure. The western suite of mafic layered gneisses are inferred to be metamorphosed oceanic arc rocks; the eastern suite of biotite schist, quartzite and calc-silicate gneiss are inferred to be metamorphosed continental sedimentary rocks. Three broadly Cretaceous, plutonic and meta-plutonic complexes record the presence of the boundary at greater depth. The Hazard Creek complex, west of the supracrustal boundary, is comprised of epidote-bearing intrusives. The Little Goose Creek complex is comprised primarily of the porphyritic orthogneiss that intruded the supracrustal boundary. The Payette River complex, east of the supracrustal boundary, is comprised of large bodies of tonalite and granite.

Each complex has a distinct geochemical character. The Hazard Creek complex is dominantly a tonalite-trondhjemite suite characterized by [...] less than .7045, [...] less than 8.4, high Sr, Na2O and Al2O3 concentrations and low MgO, Rb and K2O concentrations. Porphyritic orthogneiss in the Little Goose Creek complex has a remarkable range in [...] and [...] (.7042-.7097,8.0-10.7). The porphyritic orthogneiss is interpreted as dominated by two components: one similar in composition to the Hazard Creek complex and a second, modeled as Precambrian sedimentary material, with high [...], [...] and K2O concentrations and lower Sr concentration. The Payette River complex has generally high [...] (.7076-.7100) and variable [...](7.2-10.4). The geochemical changes indicate that the supracrustal boundary is the surface expression of a steeply-dipping structure which juxtaposes oceanic-arc lithosphere against continental lithosphere. An abrupt geochemical discontinuity preserved within the porphyritic orthogneiss, near the change in supracrustal rocks, may reflect an abrupt discontinuity at depth or may be due to the juxtapositon of portions of a stratified pluton. The juxtaposition of lithospheric blocks must have occured prior to intrusion of the porphyritic orthogneiss approximately 111 Ma, and most probably, occured before 118 Ma, prior to the beginning of plutonism. No structural evidence for the initial formation of the boundary is recognized; it is proposed to form by transform faulting or by rifting followed by convergence.

Episodic or continuous deformation along the boundary began prior to 118 Ma and produced four sets of structures. The oldest structures are foliation and isoclinal folding of crystalloblastic gneisses which may have formed during rapid burial of oceanic-arc rocks west of the boundary. Compressive deformation, forming north-south striking steeply-dipping foliations and steeply-plunging lineations in the eastern portion of the Hazard Creek complex, was broadly coeval with its emplacement. Igneous foliation and lineation with similar orientation formed during emplacement of the Payette River complex around 90 Ma. The youngest penetrative deformation formed similarly oriented, mylonitic fabrics in a 10 km wide zone centered on the boundary. All but the oldest structures are inferred to have formed by flattening and vertical flow in response to east-west compression. Deformation is interpreted to represent the response of a preexisting lithospheric boundary to compressive stresses related to subduction of material to the west.